Asynchronous usage of RegNotifyChangeKeyValue is simple enough: pass it an event object and wait for the event to be signalled.
What is not so clear, however, is the correct way to cancel a notification request. For example, if a timeout has been set on the wait like so:
RegNotifyChangeKeyValue(hKey, false, REG_NOTIFY_CHANGE_LAST_SET, regEvent.SafeWaitHandle, true);
regEvent.WaitOne(TimeSpan.FromMinutes(30))
If the timeout occurs without the notify firing, this notification is left presumably pending and waiting to fire. This is worse if the notify and wait is within a loop, potentially registering many notifications (one for each expired timeout).
Do I simply close both the event handle (regEvent) and the registry key handle (hKey)? Is there anything else I need to call, e.g. directory change notifications have FindCloseChangeNotification, is there an equivalent here?
I'm currently using P/Invoke with C#, but I do not believe that should make a difference - any answer should be focused on the requirements and usage of the Windows API. This would be much the same question if I had used RegNotifyChangeKeyValue/CreateEvent/WaitForSingleObject from VC++.
Closing the open Registry key is enough. When the key is closed, the event is signaled. This is documented behavior. So, simply be sure to close the Registry key before freeing the event.
Using event structures in LabView can get confusing, especially when mixing them with a mostly synchronous workflow. My question is, when an event structure exists in one frame of a sequence, how can I force it to ignore events (e.g. mousedown on a particular button) that were triggered while the workflow is in another frame of the sequence?
Currently, the event structures only process the events at the correct frame in the sequence, but if one was triggered while the workflow is in the previous frame, it processes those too and I want it to ignore any events that weren't triggered in the frame that the event structure exists within.
http://puu.sh/hwnoO/acdd4c011d.png
Here's part of my workflow. If the mousedown is triggered while the left part is executing, I want the event structure to ignore those events once the sequence reaches it.
Instead of placing the event structure inside your main program sequence, put it in a separate loop and have it pass the details of each event to the main sequence by means of a queue. Then you can discard the details of the events you don't want by flushing the queue at the appropriate point.
Alternatively you could use a boolean control to determine whether the event loop sends event details to the queue or discards them, and toggle the boolean with a local variable from the main sequence.
You can register for events dynamically. Registration is the point in time at which the event structure starts enqueueing events, and in your case this happens when the VI the event structure is in enters run mode (meaning it's executing or one of its callers is). You can change it so that you register using the Register for Events node and then you would only get events from that point on. When you unregister you will stop getting events.
There's a very good presentation by Jack Dunaway going into some details about events here.
You can find the code for it here.
In LabVIEW 2013 and later there are additional options for controlling the events queue, but I won't go into them here.
http://puu.sh/hwsBE/fe50dee671.png
I couldn't figure out how to flush the event queue for built-in event types like mousedown, but I managed to get around that by creating a static reference to the VI and setting the cursor to busy during the previous sequence, disabling clicking. Then when the sequence for the event structure is reached, I unset the cursor from busy, which re-enables clicking.
I have a DirectShow filter written in Delphi 6 using the DSPACK component library. It is a push source video filter that receives its source frames from an external cooperating process that I also wrote.
When the worker thread that calls my Filters' FillBuffer() call is created and ran, when the graph starts up, the first thing I do from that worker thread is create a hidden window using AllocateHWND() to process WM_COPYDATA messages that contain the externally generated frames. Right before the thread is destroyed I destroy the hidden window. In other words the hidden window is created and destroyed in the execution context of the worker thread that calls FillBuffer(). My intention is to let FillBuffer() block as it waits for a WM_COPYDATA or a WM_QUIT message. The external cooperating process will submit frames to my filter using a WM_COPYDATA message and the handle to my hidden windows' WndProcc(). I will post a WM_QUIT message in my override of the pin's Inactive() method (thanks for that tip #RomanR), to unblock the FillBuffer() call before the filter is shut down.
My question is, is it safe to call PeekMessage() or GetMessage() from the FillBuffer() call given this scenario? Or are there potential pitfalls that may arise from this occurring in the context of a DirectShow graph executing? Also, do you see any flaws in my overall approach here that I need to consider?
Safe, but not so reasonable too. FillBuffer is being called on a background worker thread which is typically have no windows on it. It would be perhaps only your window which you are going to implement message loop for. And the window is here only for the purpose of receiving WM_COPYDATA messages. It sounds like it can work out, but you would perhaps could do much easier without helper windows by passing your data between applications via named file mappings and events. In case of video (you have audio, right?) you would also be able to appreciate a smaller performance overhead.
Does anyone know where is the best place to forward thread messages to child controls?
Basically, app has to catch registered thread messages in range msg_frst...msg_last and forward them to active frame/view.
There are 3 overload points in MFC, or so it seems:
DefWndProc
OnWndMessage
PreTranslateMessage
Which is the correct one to override?
A message posted with PostThreadMessage() has a NULL window handle. So forget about any of the window methods, DispatchMessage() isn't going to deliver them. All you got is CWinThread::PreTranslateMessage().
But there's a big hazard here, this isn't going to be called anymore when any code in that thread start pumping its own message loop. Your messages now fall in the bit bucket because other code is now calling Peek/GetMessage(). That's a lot more likely then you think, MessageBox() is enough. Or the COM modal loop. Etcetera.
You can only safely use PostThreadMessage() to send messages to a thread that does not create any windows of its own. The workaround is simple enough, provide the thread with a window handle so it can call PostMessage() instead. That could be a hidden window, dedicated to handling these messages for example.
The documentation for the Win32 API PulseEvent() function (kernel32.dll) states that this function is “… unreliable and should not be used by new applications. Instead, use condition variables”. However, condition variables cannot be used across process boundaries like (named) events can.
I have a scenario that is cross-process, cross-runtime (native and managed code) in which a single producer occasionally has something interesting to make known to zero or more consumers. Right now, a well-known named event is used (and set to signaled state) by the producer using this PulseEvent function when it needs to make something known. Zero or more consumers wait on that event (WaitForSingleObject()) and perform an action in response. There is no need for two-way communication in my scenario, and the producer does not need to know if the event has any listeners, nor does it need to know if the event was successfully acted upon. On the other hand, I do not want any consumers to ever miss any events. In other words, the system needs to be perfectly reliable – but the producer does not need to know if that is the case or not. The scenario can be thought of as a “clock ticker” – i.e., the producer provides a semi-regular signal for zero or more consumers to count. And all consumers must have the correct count over any given period of time. No polling by consumers is allowed (performance reasons). The ticker is just a few milliseconds (20 or so, but not perfectly regular).
Raymen Chen (The Old New Thing) has a blog post pointing out the “fundamentally flawed” nature of the PulseEvent() function, but I do not see an alternative for my scenario from Chen or the posted comments.
Can anyone please suggest one?
Please keep in mind that the IPC signal must cross process boundries on the machine, not simply threads. And the solution needs to have high performance in that consumers must be able to act within 10ms of each event.
I think you're going to need something a little more complex to hit your reliability target.
My understanding of your problem is that you have one producer and an unknown number of consumers all of which are different processes. Each consumer can NEVER miss any events.
I'd like more clarification as to what missing an event means.
i) if a consumer started to run and got to just before it waited on your notification method and an event occurred should it process it even though it wasn't quite ready at the point that the notification was sent? (i.e. when is a consumer considered to be active? when it starts or when it processes its first event)
ii) likewise, if the consumer is processing an event and the code that waits on the next notification hasn't yet begun its wait (I'm assuming a Wait -> Process -> Loop to Wait code structure) then should it know that another event occurred whilst it was looping around?
I'd assume that i) is a "not really" as it's a race between process start up and being "ready" and ii) is "yes"; that is notifications are, effectively, queued per consumer once the consumer is present and each consumer gets to consume all events that are produced whilst it's active and doesn't get to skip any.
So, what you're after is the ability to send a stream of notifications to a set of consumers where a consumer is guaranteed to act on all notifications in that stream from the point where it acts on the first to the point where it shuts down. i.e. if the producer produces the following stream of notifications
1 2 3 4 5 6 7 8 9 0
and consumer a) starts up and processes 3, it should also process 4-0
if consumer b) starts up and processes 5 but is shut down after 9 then it should have processed 5,6,7,8,9
if consumer c) was running when the notifications began it should have processed 1-0
etc.
Simply pulsing an event wont work. If a consumer is not actively waiting on the event when the event is pulsed then it will miss the event so we will fail if events are produced faster than we can loop around to wait on the event again.
Using a semaphore also wont work as if one consumer runs faster than another consumer to such an extent that it can loop around to the semaphore call before the other completes processing and if there's another notification within that time then one consumer could process an event more than once and one could miss one. That is you may well release 3 threads (if the producer knows there are 3 consumers) but you cant ensure that each consumer is released just the once.
A ring buffer of events (tick counts) in shared memory with each consumer knowing the value of the event it last processed and with consumers alerted via a pulsed event should work at the expense of some of the consumers being out of sync with the ticks sometimes; that is if they miss one they will catch up next time they get pulsed. As long as the ring buffer is big enough so that all consumers can process the events before the producer loops in the buffer you should be OK.
With the example above, if consumer d misses the pulse for event 4 because it wasn't waiting on its event at the time and it then settles into a wait it will be woken when event 5 is produced and since it's last processed counted is 3 it will process 4 and 5 and then loop back to the event...
If this isn't good enough then I'd suggest something like PGM via sockets to give you a reliable multicast; the advantage of this would be that you could move your consumers off onto different machines...
The reason PulseEvent is "unreliable" is not so much because of anything wrong in the function itself, just that if your consumer doesn't happen to be waiting on the event at the exact moment that PulseEvent is called, it'll miss it.
In your scenario, I think the best solution is to manually keep the counter yourself. So the producer thread keeps a count of the current "clock tick" and when a consumer thread starts up, it reads the current value of that counter. Then, instead of using PulseEvent, increment the "clock ticks" counter and use SetEvent to wake all threads waiting on the tick. When the consumer thread wakes up, it checks it's "clock tick" value against the producer's "clock ticks" and it'll know how many ticks have elapsed. Just before it waits on the event again, it can check to see if another tick has occurred.
I'm not sure if I described the above very well, but hopefully that gives you an idea :)
There are two inherent problems with PulseEvent:
if it's used with auto-reset events, it releases one waiter only.
threads might never be awaken if they happen to be removed from the waiting queue due to APC at the moment of the PulseEvent.
An alternative is to broadcast a window message and have any listener have a top-level message -only window that listens to this particular message.
The main advantage of this approach is that you don't have to block your thread explicitly. The disadvantage of this approach is that your listeners have to be STA (can't have a message queue on an MTA thread).
The biggest problem with that approach would be that the processing of the event by the listener will be delayed with the amount of time it takes the queue to get to that message.
You can also make sure you use manual-reset events (so that all waiting threads are awaken) and do SetEvent/ResetEvent with some small delay (say 150ms) to give a bigger chance for threads temporarily woken by APC to pick up your event.
Of course, whether any of these alternative approaches will work for you depends on how often you need to fire your events and whether you need the listeners to process each event or just the last one they get.
If I understand your question correctly, it seems like you can simply use SetEvent. It will release one thread. Just make sure it is an auto-reset event.
If you need to allow multiple threads, you could use a named semaphore with CreateSemaphore. Each call to ReleaseSemaphore increases the count. If the count is 3, for example, and 3 threads wait on it, they will all run.
Events are more suitable for communications between the treads inside one process (unnamed events). As you have described, you have zero ore more clients that need to read something interested. I understand that the number of clients changes dynamically. In this case, the best chose will be a named pipe.
Named Pipe is King
If you need to just send data to multiple processes, it’s better to use named pipes, not the events. Unlike auto-reset events, you don't need own pipe for each of the client processes. Each named pipe has an associated server process and one or more associated client processes (and even zero). When there are many clients, many instances of the same named pipe are automatically created by the operating system for each of the clients. All instances of a named pipe share the same pipe name, but each instance has its own buffers and handles, and provides a separate conduit for client/server communication. The use of instances enables multiple pipe clients to use the same named pipe simultaneously. Any process can act as both a server for one pipe and a client for another pipe, and vice versa, making peer-to-peer communication possible.
If you will use a named pipe, there would be no need in the events at all in your scenario, and the data will have guaranteed delivery no matter what happens with the processes – each of the processes may get long delays (e.g. by a swap) but the data will be finally delivered ASAP without your special involvement.
On The Events
If you are still interested in the events -- the auto-reset event is king! ☺
The CreateEvent function has the bManualReset argument. If this parameter is TRUE, the function creates a manual-reset event object, which requires the use of the ResetEvent function to set the event state to non-signaled. This is not what you need. If this parameter is FALSE, the function creates an auto-reset event object, and system automatically resets the event state to non-signaled after a single waiting thread has been released.
These auto-reset events are very reliable and easy to use.
If you wait for an auto-reset event object with WaitForMultipleObjects or WaitForSingleObject, it reliably resets the event upon exit from these wait functions.
So create events the following way:
EventHandle := CreateEvent(nil, FALSE, FALSE, nil);
Wait for the event from one thread and do SetEvent from another thread. This is very simple and very reliable.
Don’t' ever call ResetEvent (since it automatically reset) or PulseEvent (since it is not reliable and deprecated). Even Microsoft has admitted that PulseEvent should not be used. See https://msdn.microsoft.com/en-us/library/windows/desktop/ms684914(v=vs.85).aspx
This function is unreliable and should not be used, because only those threads will be notified that are in the "wait" state at the moment PulseEvent is called. If they are in any other state, they will not be notified, and you may never know for sure what the thread state is. A thread waiting on a synchronization object can be momentarily removed from the wait state by a kernel-mode Asynchronous Procedure Call, and then returned to the wait state after the APC is complete. If the call to PulseEvent occurs during the time when the thread has been removed from the wait state, the thread will not be released because PulseEvent releases only those threads that are waiting at the moment it is called.
You can find out more about the kernel-mode Asynchronous Procedure Calls at the following links:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms681951(v=vs.85).aspx
http://www.drdobbs.com/inside-nts-asynchronous-procedure-call/184416590
http://www.osronline.com/article.cfm?id=75
We have never used PulseEvent in our applications. As about auto-reset events, we are using them since Windows NT 3.51 (although they appeared in the first 32-bit version of NT - 3.1) and they work very well.
Your Inter-Process Scenario
Unfortunately, your case is a little bit more complicated. You have multiple threads in multiple processes waiting for an event, and you have to make sure that all the threads did in fact receive the notification. There is no other reliable way other than to create own event for each consumer. So, you will need to have as many events as are the consumers. Besides that, you will need to keep a list of registered consumers, where each consumer has an associated event name. So, to notify all the consumers, you will have to do SetEvent in a loop for all the consumer events. This is a very fast, reliable and cheap way. Since you are using cross-process communication, the consumers will have to register and de-register its events via other means of inter-process communication, like SendMessage. For example, when a consumer process registers itself at your main notifier process, it sends SendMessage to your process to request a unique event name. You just increment the counter and return something like Event1, Event2, etc, and creating events with that name, so the consumers will open existing events. When the consumer de-registers – it closes the event handle that it opened before, and sends another SendMessage, to let you know that you should CloseHandle too on your side to finally release this event object. If the consumer process crashes, you will end up with a dummy event, since you will not know that you should do CloseHandle, but this should not be a problem - the events are very fast and very cheap, and there is virtually no limit on the kernel objects - the per-process limit on kernel handles is 2^24. If you are still concerned, you may to the opposite – the clients create the events but you open them. If they won’t open – then the client has crashed and you just remove it from the list.